drm/amdgpu: use only one reservation object for each VM v2

Reduces the locking and fencing overhead.

v2: add comment why we need the duplicates list in the GEM op.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>